rtlwifi: Add encryption argument in rtl_is_special_data for checking DHCP packet.

rtl8192cu can't connect to AP after physical reconnect.
according to dmesg, that problem's cause was DHCP timeout.

rtl_is_special_data function checks packet type for adjusting rate.
when that function is called from _rtl_rc_get_highest_rix, it can not
calculate offset correctly. so i add argument is_encn in rtl_is_special_data.
is_enc variable mean that iv header is added in skb parameter.

i test only rtl8192cu chipset. because i doesn't have other rtlwifi chipsets.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>